Hey, Runatic!
On October 17, 2004, Xu Zhenjun completed a marathon in Beijing in 3:43:39, running backward the entire way. Guinness recorded the result in its backward-running category. The record story
That's roughly 5:18 per kilometer for 42.195 km.
He didn't just break four hours. He spent most of the time looking at where he'd come from.
Most marathoners have the finish ahead of them. Xu kept it behind him until the very end. In case an ordinary marathon wasn't complicated enough.